1. Field of the Invention
The present invention relates to wireless networks, more particularly, to a method for maximizing the efficiency of a network handoff in a public wireless local area network (PWLAN) environment.
2. Description of the Prior Art
Advances and improvements in wireless communication technology have lead to an enormous increase in wireless usage over the past years. The popularity of wireless devices comes from their ease of use, their versatility, the geographical freedom they provide, and the vast amounts of information they are able to seamlessly exchange. Wireless users have the ability to choose from a multitude of wireless endpoints, including: cellular telephones, personal desktop assistants (PDAs), global positioning systems (GPS), pagers, and compact notebook computers. Each of these different endpoints typically connect to and communicate through a different network protocols. These wireless communications protocols may include: GSM, Bluetooth, WiFi (Wireless Fidelity), and WiMax.
One of the more popular types of regional network configurations is the public wireless local area network (PWLAN) utilizing an IEEE 802.11 protocol. This configuration allows a user to operate and communicate through a wireless endpoint (typically a notebook computer) in a semi macroscopic region. The increase and popularity in the usage of PWLANs have lead to rising demands for a seamless handoff across different networks in a PWLAN environment. When a wireless device switches communication from one network provider to another in a wireless environment of the same protocol, a certain latency time is associated with the horizontal handoff. Although this latency time may be acceptable for certain low bandwidth processes, such as Internet surfing, high bandwidth and real time processes are seriously hindered and delayed. For example, if a long latency time is experienced during a network handoff while involved in a voice over IP (VOIP) process, the performance and continuity of sound would be seriously interrupted, with lost segments in speech and disconnection possible.
Before commencing with a network handoff, a wireless endpoint performs a local scanning procedure to identify possible providers to connect to. Typically, the endpoint attempts to identify and connect to the provider with the greatest signal to noise ratio (SNR) to allow for the clearest and strongest transmissions. The active scanning performed by the wireless endpoint generally consists of two stages, a probe request, and a probe response. As a wireless endpoint moves from one wireless access region to another, it begins sending requests to the alternate wireless access points to initiate communication in the new region. Before commencing with communication, it must wait for a receipt of a probe response from the access point.
FIG. 1 is an exemplary diagram illustrating the probe request/response sequence performed by the wireless endpoint. In FIG. 1, the wireless endpoint consists of a notebook computer at a first position 100, and the same notebook computer at a second position 110. While in the first position 100 the wireless notebook is in the communication proximity of the current access point 120, and while in the second position 110, the wireless notebook is in the communication proximity of the neighboring access point 130. As the notebook computer moves from the first position 100 to the second position 110, it begins to lose a sufficient SNR with the current access point 120, and gain a strong SNR with the neighboring access point 130. As such, the wireless endpoint begins a network scan by sending probe requests to the neighboring access point 130 to locate the provider with the strongest SNR. If the probe request is received by the neighboring access point 130, it is acknowledged with a probe response. If the probe response is received by the wireless notebook, it will send an acknowledgement packet to the access point, and will be in communication with this provider as it reaches the second position 110, as utilizing the neighboring access point 130 in the current region would provide the greatest SNR.
Merely sending a probe request, however, does not automatically guarantee receiving a probe response. Many times, a probe request may not be received by the access point for various reasons. The probe request transmission may have its path blocked by an inanimate object, the request may be outside of the receiving range of the access point, or the probe request frame may collide with data frames from the existing users in the network. Therefore, oftentimes multiple probe requests have to be sent by the wireless endpoint before receiving a successful probe response. It is only upon the receipt of a successful probe request that a wireless endpoint can initiate communications through the access point.
The sending of probe requests and probe responses is therefore divided in time into “periods”. Because of the potentially low probability of a single probe request receiving a successful probe response, multiple frames of probe requests and responses are sent within an entire period. For example, one probe request period may contain 3 individual probe request frames. Since the wireless endpoint sends out requests as an integer number of periods, the amount of individual probe request frames it can send in this case is 3 (1 probe request period). Each period contains a specific number of probe request frames or probe responses frames as determined by the system, however each individual probe request (or response) must be separated by a DIFS which acts as a buffer to space the frames in time.
FIG. 2 illustrates in depth the constitution of a period and the entire scanning process in accordance to IEEE 802.11 WLAN protocol. A probe request period 202 comprises a DIFS buffer separation 204 to provide spacing in time between signals, a contention delay 206, and a probe request frame 208. As illustrated in FIG. 2, the probe request period 202 in this example comprises two different probe request frames 208. The number of probe request frames 208 in a probe request period 202 is known as the broadcast count (mreq). Once a probe request period 202 is sent out, the access point waits for its receipt (indicated by the vertical dashed line) before sending out a probe response period 212. Similar to the construction of the probe request period 202, the probe response period 212 comprises a DIFS buffer separation 204, a contention delay 206, and a probe response frame 210. The number of probe response frames 210 in a probe response period 212 is known as the retry count (m′). Probe response periods 212 are thus continually sent back to the wireless endpoint until an acknowledgement frame 214 is successfully sent by the wireless endpoint back to the access point. The acknowledgement frame 214 is thus the confirming handshake that enables communication between the wireless endpoint and access point to begin. The total time required for this entire process from sending a probe request frame 208, to receiving an acknowledgement frame 214 is called the search latency (tb). Finally, the minimum probe response time 220 is the time it takes for a mobile station to locate an access point in an idle channel, meaning a channel without access points in it. The maximum probe response time 230 is the time it takes an access point to receive an acknowledgement frame 214 upon the receipt of a probe request frame 208.
A high search probability (Ps—search) is generally desired because it would increase the chances of locating an access point within one channel searching process step (one transmission of a probe request period and probe response period as shown in FIG. 2). However, as this process is statistical, if an access point is not located within the single transmission, another probe request period will have to be transmitted, adding further delay to the handoff process.
In order to increase the probability of a successful search (Ps—search) for an access point, the broadcast count (mreq) of the probe request period 202 can be increased. This will increase the number of probe request frames 208 sent within a period and increase the odds of a successful reception in the event that some packets may not be received. However, increasing the broadcast count (mreq) is not without its disadvantages. Sending more probe request frames 208 will result in a longer probe request period 202. The additional probe request frames 208 will also require additional contention delays 206 and DIFS buffer separations 204 which will further lengthen the probe request period 202. The main parameter affected by this is the search latency (tb), which may result in being unnecessarily long before a successful handoff can be confirmed.
For similar reasons, the retry count (m′) of the probe response period 212 can also be increased to raise the probability of a successful search. A wireless endpoint must receive the response frame 210 from the access point before it can send out an acknowledgement frame 214. Therefore, increasing the retry count (m′) of the probe response period 212 will also increase the odds of a successful reception, but will also equally increase the search latency (tb) of the handoff process.
Reducing the broadcast count (mreq), or retry count (m′) will surely reduce the search latency (tb) delay, as the probe request period 202 and probe response period 212 will be shortened. However, due to the reduced number of request packets 208 and response packets 210 sent within a single period, a low probability of a successful search may result. Therefore the probability a successful handoff within a single period is further reduced.